Phase shift devices are capable of introducing a change in phase, or generally a time delay, in an optical fiber signal and are mainly used in the communication field, with those systems where mutually phase-shifted signals are required to be processed.
Particularly, devices are known which change the phase of a signal travelling through an optical fiber by means of an elasto-optical effect. This effect provides that the refractive index of the fiber core is changed by means of a mechanical action (either of elongation or stress) applied on the fiber. Changing the refractive index implies changing the optical path seen by the signal, the latter thus experiencing a different phase shift from prior to changing the refractive index.
An example of a phase shifter employing the elasto-optical effect is disclosed in U.S. Pat. No. 4,814,774. This document describes a phased-controlled antenna system and comprising: a laser source, a signal modulator, a splitter, a plurality of optical fiber connections, a phase controller and an array of antenna elements.
The optical signals reaching each element in the array have such mutual phase relationships that the radiation beam emitted from the array antenna has a predetermined shape and a predetermined orientation. The mutual phase shift among the signals of each connections can be introduced and changed using the phase controller, which operates by means of elasto-optical effect thereby controlling cylindrical piezoelectric crystals having the individual fibers coiled thereon. The piezoelectric crystals, after an electric voltage has been applied thereto, expand their circumference, and consequently cause a stress on the coils in which the fiber is wound, thereby changing the refractive index of the latter.
The phase delay introduced on each fiber connection depends on the number of windings and the stress action to which the fiber is subjected by the voltage-controlled piezoelectric crystal. Thereby, an optical beam scanning is outputted from the array antenna. In this document of the prior art, magnetoresistive and electro-mechanical elements are mentioned alternatively to the piezoelectric crystals proposed.
This type of devices, which operate by elasto-optical effect, are difficult to control and, following their operation, the phase delay introduced in the fiber is not observed in a sufficiently short time. Furthermore, manufacturing these devices is relatively complicated.